The present invention applies to aircraft guidance, such as the guidance of transport airplanes in particular, during the terminal phase of a landing, that is, during the final deceleration phase before landing on a landing runway. The object of the present invention is to assure the aircraft of a given time of passage, of RTA (required time of arrival) type, at a particular waypoint that is situated on the approach trajectory followed during this approach phase, notably at the point of the approach trajectory, which is situated at the level of the upstream threshold of the landing runway.
Document FR-2 913 780 discloses a method and a device for assisting in the guidance of an aircraft, the object of which is to make the aircraft observe a target time of passage at a given point. This point can notably be situated in a landing phase. It may, in particular, be a point of convergence at which the different aircraft (which are expecting to land at one and the same airport) converge before performing the final landing phase.
Moreover, it is known that many flight management systems (FMS) contain a particular function that allows speed servocontrol to be implemented in order to enable the aircraft to meet a time constraint, of RTA type, at a particular point of the flight.
Such speed servocontrol is, however, performed only during cruising and descent phases. It is known that a descent phase is the flight phase between, on the one hand, the cruising phase, and, on the other hand, the approach phase considered in the present invention.
Particularly because of strict guidance constraints during the terminal landing phase, such common speed servocontrol is not implemented in the approach phase. In the context of the present invention, it is assumed that the approach phase corresponds exclusively to the phase during which the aircraft decelerates from a so-called descent speed, for example 250 knots (approximately 130 meters per second), to a so-called approach speed which corresponds to the speed at which the aircraft must overfly the upstream threshold of the runway used for the landing.
Consequently, the usual solutions do not make it possible to ensure an exact time of passage at any point of the approach trajectory during the approach phase, notably at the threshold of the landing runway, because no flexibility is allowed in relation to the flight time for the approach phase. In practice, the flight time in relation to the approach phase is always the same, since, usually, the duration of the approach phase corresponds precisely to the time needed to decelerate from the descent speed to the approach speed.
More specifically, to servocontrol the aircraft speed-wise during this approach phase, the following operations are usually carried out:    A/ a speed profile is determined which illustrates a variation of the speed of the aircraft as a function of the distance of said aircraft relative to the threshold of the landing runway used for the landing;    B/ speed set points are calculated to enable the aircraft to observe this speed profile; and    C/ during the approach phase, said speed set points are applied to the aircraft, while guiding it along said approach trajectory.
This usual speed servocontrol is therefore performed without taking into account a time constraint, of RTA type. Consequently, a delay or an advance of the aircraft relative to a desired time of passage above the threshold of the runway, that is to say relative to its landing time, cannot be corrected during the approach phase.